Plasma arc cutting is a process in which an electric arc is used to cut a metallic workpiece. Generally, plasma arc cutting uses an electric arc between an electrode and the metal to be cut. The arc creates a plasma that cuts the metallic workpiece.
It is generally accepted that approximately 250 volts (open circuit) is desirable to initiate a plasma arc cutting process. After the process has been initiated, the cutting arc voltage is approximately 90-125 volts. Of course, the cutting arc (load output) voltage is dependent upon the length of the cutting arc. The greater the length of the arc, the greater the load voltage, and conversely, the lesser the length of the arc the lesser the load voltage. Similarly, the cutting arc voltage varies with the magnitude of the output current.
A typical prior art plasma arc cutting power supply receives an input voltage (from a power line or generator) and provides an output voltage to a pair of output terminals, one of which is the electrode and the other of which is connected to the workpiece. The power supplies provide about 250 volts open circuit and about 90-125 volts under load. There are numerous types of known plasma arc cutting power supplies, such as magnetic power supplies, inverter power supplies, phase control power supplies, and choppers or secondary switchers. The present invention relates to chopper based power supplies.
A typical prior art chopper shown in FIG. 5 includes a voltage source 601, a switch 602, a diode 604 and an inductor 603 which provide output current to load 605. Voltage source 601 may be a transformer receiving line voltage and a rectifier. Voltage source 601 should be capable of providing input power, generally at a desired voltage level. Of course, the source need not be a constant voltage source but merely should be suitable for use with a chopper power supply. When switch 602 is on current flows from the positive output of voltage source 601 through load 605, inductor 603 and switch 602. During this time the load current (and inductor current) is increasing and inductor 603 is storing energy, dependent upon the volt seconds applied to inductor 603 while 602 is on. The volt seconds are determined by the time switch 602 is on, and the source and load voltages. When switch 602 is off current freewheels through diode 604, load 605 and inductor 603. While switch 602 is off the load current (and inductor current) is decreasing and inductor 603 is returning energy, again dependent upon the volt seconds (the time and load voltage) inductor 603 applies to diode 604 and load 605. In some high current applications a single voltage source may have more than one chopper connected in parallel. The choppers are operated in-phase with one another, thus the load current is merely twice the output current of either chopper.
The output voltage applied to load 605 is dependent upon the duration of time switch 602 is on, relative to the duration of time switch 602 is off. Specifically, the output voltage is equal to the ratio of on time of switch 602 to the on time plus off time of switch 602, multiplied by the output voltage of voltage source 601. Thus, if switch 602 is on 50% of the time the load voltage will be 50% of the source voltage. Typically, a current feedback element is used in conjunction with a control circuit to control the on and off time of switch 602.
Thus, a chopper will have a triangle (sawtooth) current output having predetermined, but variable amplitude or frequency according to the switching frequency. Their operation is based on the controlled switching of a DC voltage input to a desirable DC current output. The sawtooth output may be characterized as having a ripple current, dependent on the maximum current less the minimum current.
One drawback of chopper power supplies is the ripple current rating of the output. When in-phase parallel choppers are used, the output ripple is twice the ripple of a single chopper. In a switching type (chopper) power supply, the output cutting capability of the air plasma arc cutter is adversely affected if its ripple current capability is not adequate for the job, i.e., cutting occurs best at relatively constant current, not with a sawtooth output. The life span of input capacitors in the voltage source is also affected if the ripple current is greater than the ratings. The ripple current generates internal heat in the capacitor, with the attendant changes in temperature dependent parameters. Elevated temperatures may reduce the life expectancy of any electrochemical component. It has often proved difficult to ensure a moderate ambient temperature for capacitors, much less to aggravate the situation by permitting excessive ripple currents. Even with an appropriately rated capacitor, dangerous internal temperatures can develop when there is no provision for heat removal from the external surface of the case. Additionally, the magnitude of the peak current is dependent upon the percent of ripple. Because high peak current can erode consumables, a low ripple current is desirable.
Chopper power supplies, however, are relatively inexpensive, controllable, and not lossy. Also, choppers are well suited for receiving an input voltage, and provide a load current at a lesser output voltage. Accordingly, it is desirable to provide a chopper based power supply to take advantage of the positive aspects of choppers, yet avoid a major drawback--excessive ripple current in the load.
Welding power supplies have many similarities to plasma arc cutting power supplies. Specifically, the welding process is best initiated with a generally accepted fixed open circuit voltage (approximately 80 volts). After initiation, however, the operating load output voltage is generally in the range of 20-45 volts, and often 25 volts. Finally, as in the plasma arc cutting process the actual arc voltage varies with the current and the length of the arc.
In some welding applications it is desirable to have a single power supply provide current to a number of welding stations connected in parallel. Thus, more than one welder can use a given power supply with this sort of arrangement. Typically, to provide the necessary open circuit voltage to initiate the welding arc the power supply will be a constant voltage, 80 volts source. Each welding station includes a variable resistor in series with the welding electrode (or workpiece). The resistor dissipates sufficient power to provide a typical load output voltage, 20-45 volts, e.g. As may be readily seen this is very wasteful of power--as little as 25% of the power is delivered to the arc load, while 75% of the power is dissipated in the resistor.
Accordingly, it is desirable to provide a welding power supply that is capable of providing 80 volts open circuit and in the range of 25 volts load voltage. Preferably, such a welding power supply would be capable of providing multiple parallel welding stations, without wasting power.